1. Field of the Invention
The present invention relates to developing devices and image forming apparatuses, and more particularly to developing devices and image forming apparatuses of a non-magnetic one-component developing type.
2. Description of Related Art
In a developing device for an image forming apparatus, in order to avoid adhesion of grease to toner, desirably, grease is not applied to bearings of a developing roller. Therefore, as the developing roller, a solid shaft made of metal, such as iron, is used, and the solid shaft is machined such that its both ends to be supported by bearings become narrower than its center for delivering a developer. With this configuration, the developing roller has a smaller inertia, which permits a motor for rotating the developing roller to have a reduced driving torque and which eliminates the necessity of applying grease to the bearings of the developing roller. Additionally, both ends of the developing roller, which are supported by the bearings, are subjected to a mirror surface treatment so that friction between the bearings and the both ends of the developing roller can be reduced, which eliminates the necessity of applying grease to the bearings of the developing roller. Moreover, as the bearings, for example, ball bearings are used, which reduces the friction between the both ends of the developing roller and the bearings and which eliminates the necessity of applying grease to the bearings of the developing roller.
Image forming apparatuses of a non-magnetic one-component developing type are simpler than image forming apparatuses of a two-component developing type and are mostly used for inexpensive systems. Therefore, it is especially demanded that developing devices to be employed in image forming apparatuses of a non-magnetic one-component developing type be manufactured at low cost. As will be described in the following, however, various trials to reduce the manufacturing cost of developing devices result in an increase of the driving torque of the motor for rotating the developing roller.
Specifically, in order to reduce the manufacturing cost, for example, resin bearings, instead of the ball bearings, may be used. In this case, however, the developing roller has larger friction between its both ends and the resin bearings when rotating, compared with the case of using the ball bearings, and the motor for rotating the developing roller is required to have a larger driving torque.
Another way of reducing the manufacturing cost may be using an inexpensive aluminum hollow pipe, instead of the iron solid shaft, as the developing roller. In this case, the aluminum pipe is subjected to drawing such that its both ends to be supported by bearings become narrower than its center for delivering a developer and is subjected to machining for shaping, so that the aluminum developing roller has a reduced inertia.
It is, however, difficult to narrower the both ends of the aluminum pipe by drawing, compared with the case of narrowing the both ends of the solid developing roller by machining. Therefore, the aluminum pipe developing roller has a larger inertia and accordingly requires the motor to have a larger driving torque, compared with the solid developing roller.
Further, in order to reduce the manufacturing cost, the both ends of the aluminum pipe developing roller are subjected to machining but are not subjected to a mirror surface treatment or superoll processing. Therefore, the surface roughness of the aluminum pipe developing roller is higher than that of a developing roller subjected to a mirror surface treatment or superoll processing. Accordingly, the aluminum pile developing roller has larger friction between its both ends and the bearings than a developing roller subjected to a mirror surface treatment or superoll processing. Consequently, the motor is required to have a larger driving torque.
As described above, combination of various measures to reduce the manufacturing cost of the developing device results in an increase in the driving torque of the motor. When an inexpensive motor is used for rotating the developing roller, the motor may come into step-out due to its insufficient driving torque. As well as the step-out of the motor, the connection between the motor and the developing roller may be out of gear. In order to avoid such trouble, in a developing device with the above-described measures to reduce the manufacturing cost taken, it is necessary to apply grease to between the bearings and the both ends of the developing roller.
On the other hand, even if grease is applied to between the bearings and the both ends of the developing roller, the grease will be used out as the developing device is used. The use-out of grease increases the friction between the bearings and the developing roller, which requires the motor to have a larger driving torque. Consequently, the motor may come into step-out, and/or the connection between the motor and the developing roller may be out of gear.
Known examples having a structure for preventing use-out of grease between a shaft and bearings are a motor disclosed by Japanese Patent Laid-Open Publication No. 9-103043 and a recorder disclosed by Japanese Patent Laid-Open Publication No. 2004-338105. More specifically, Japanese Patent Laid-Open Publication No. 9-103043 and Japanese Patent Laid-Open Publication No. 2004-338105 disclose that a groove is made in the shaft and that grease is pooled in the groove. Japanese Patent Laid-Open Publication No. 9-103043 relates to a motor, and Japanese Patent Laid-Open Publication No. 2004-338105 relates to a recorder. Thus, these publications relate to technical fields totally different from the field of the present invention, i.e., the field of developing devices, and neither of these publications describes measures to prevent use-out of grease.